1. Field of the Invention
The present invention relates to a semiconductor sensor, such as a pressure sensor, acceleration sensor or the like, which utilizes a semiconductor element as a sensing means and, more particularly, to a sealed package with a cap (also known as a hermetically sealed package) structure for a semiconductor sensor.
2. Description of the Prior Art
FIG. 7 is a front view of a semiconductor sensor having a conventional can-sealed package structure. As a hermetically sealed structure for the semiconductor sensor, a so-called cap sealed package structure has been well known and comprises a mounting plate 3 called stem and a box-shaped cap 2 with a mounting flange welded on the stem 3, as shown in FIG. 7. Two holes 10 for mounting the semiconductor sensor 1 on a mounting plate (not shown) are provided at both round ends of the stem 3 and a plurality of electrical leads 4 protrude downwardly from the stem 3, passing therethrough.
As shown in FIG. 8 showing a plan view of the semiconductor sensor before welding of the cap 2, a semiconductor sensor element 5 is mounted on a hybrid IC circuit board 6 fixed on the stem 3 and each of terminals arranged along one side of an HIC circuit board 6 is electrically connected to a corresponding lead 4 by a bonding wire 7. An area of the stem 3 including HIC circuit board 6 and leads 4 is covered and sealed by the cap 2 when welded.
The structure for inserting and fixing the leads 4 to the stem 3 in a manner sealing the inside of the cap 2 shown in FIG. 7 is described next below.
FIG. 9 is an enlarged view of an area at which one terminal of the HIC circuit board 6 and one lead 4 are connected by means of the bonding wire 7 as shown in FIG. 8. The conductive leads 4 are inserted into the stem 3 before the HIC circuit board 6 is fastened to the stem 3. A glass tube is first fit onto each lead 4 to cover that part of the conductive lead 4 inserted to the stem 3, and the glass tube and conductive lead 4 subassembly is then inserted into a lead insertion hole of the stem 3. This subassembly is then heated to melt the glass tubes, thus forming a glass seal 8 around each lead 4 as shown in FIG. 9, securing and sealing each of the leads 4 in the lead insertion hole of the stem 3. The inside of the cap 2 is then sealed by welding the cap 2 to the stem 3.
With the semiconductor sensor 1 of a type shown in FIGS. 7 and 8, however, the semiconductor sensor 1 is secured to a mounting plate (not shown) with screws passing the mounting holes 10. In order to provide these mounting holes 10, the stem 3 has to have extended areas at both ends thereof in a length-wise direction of the stem and the size of the stem 3 necessarily increases to provide the mounting holes 10, thus increasing the size and overall cost of the semiconductor sensor unit.
The semiconductor sensor 1a shown in FIG. 10 was therefore developed to solve this problem. With this semiconductor sensor la, the conductive leads 4 are arranged in a balanced pattern conforming to the shape of the stem 3a, and the conductive leads 4 are soldered to the circuit board on which the semiconductor sensor 1 is mounted. FIG. 10 is a plan view of this semiconductor sensor 1a before the cap 2 is welded in place. Note, further, that like parts are identified by like reference numerals in FIGS. 8 and 10.
As shown in FIG. 10, the mounting holes 10 provided in the stem 3 shown in FIG. 8 are eliminated, the conductive leads 4 are arranged on the stem 3a equally spaced in two rows on opposite sides of the HIC circuit board 6a, and the conductive leads 4 are soldered to a mounting board to which the semiconductor sensor 1a is mounted to secure the semiconductor sensor la in place.
In the semiconductor sensor 1a shown in FIG. 10, however, when the semiconductor sensor 1a is soldered to the circuit board and when vibration or impact is applied to the solder joint after securing the semiconductor sensor 1a to the mounting board, the solder joints of the conductive leads 4 and the glass seal 8 are stressed, leading to degraded reliability in each member. Furthermore, though the semiconductor sensor having the structure shown in FIG. 10 makes it possible to eliminate mounting holes, it necessitates an increase in the width of a stem, and, therefore, it is impossible to decrease the area of the stem.